Somali pirates in the Gulf of Aden seized a Hong Kong-registered cargo ship carrying 36,000 tonnes of wheat to Iran yesterday in the latest of the near-daily attacks threatening one of the world's busiest shipping routes.

The Delight, with 25 crew on board, was captured off Yemen, the seventh successful hijacking in the past 12 days. The US navy, whose patrols along Somalia's coast appear to be having little effect on the pirates, said the ship belonged to Iran's state shipping line.

A British tanker also came under attack yesterday, but the pirates were thwarted when the German frigate Karlsruhe launched a helicopter to intercept them. Eight or nine speedboats of heavily armed bandits attacked the British tanker Trafalgar, which radioed the German ship for help. It sent a Sea King and the pirates fled, the German navy said.

The attacks came a day after it was revealed that Somali pirates had hijacked a Saudi supertanker carrying $100m (£67m) of oil. The Sirius Star, which was bound for the US, was hijacked 450 miles south-east of the Kenyan port of Mombasa and is the largest vessel ever captured by pirates. It was reported to have anchored near Harardheere, half way up Somalia's eastern shoreline. The Foreign Office said the two Britons among the 25 crew include the tanker's chief engineer and a second officer. Speaking from Kenya, the armed forces minister, Bob Ainsworth, said: "We call on those holding the men to release them and the rest of the crew immediately."

Britain had earlier handed over eight suspected Somali pirates to Kenya after it obtained assurances they would get a fair trial and would not be executed, UK diplomatic and defence officials said. The Somalis were seized after they attempted to hijack a Danish vessel in the Gulf of Aden last week. Two other suspected Somali pirates were killed by Royal Marine commandos from the frigate HMS Cumberland. The navy said they were shot in self-defence. A Yemeni national found injured later died, despite emergency treatment from Cumberland's doctor.

The Saudi foreign minister, Prince Saud al-Faisal, described the hijacking of the Sirius Star, which is carrying 2m barrels of oil, as an "outrageous act" and promised to support a European-led initiative to increase security off Africa's east coast.

"Piracy, like terrorism, is a disease which is against everybody, and everybody must address it together," he said.

Faced with an Islamist insurgency and crippled by infighting among its own ranks, Somalia's government is powerless to stop the numerous pirate groups said to be employing up to 3,000 gunmen.

Vela International, the shipping arm of the state oil company Saudi Aramco, which owns the Sirius Star, said the crew was safe and unharmed. No ransom demand had yet been received.

Somali pirates have received more than £20m in ransom payments this year, and ships have recently been fetching millions of dollars each. At least a dozen vessels and more than 250 international crew members are being held hostage. Most of the ships are anchored near Eyl, including the Ukrainian-owned MV Faina, carrying 33 Russian-made tanks.

The pirates, typically armed with AK47s and rocket-propelled grenades, quickly adapt. When ships avoided the Somali coastline they began using captured "mother ships" as a base to launch their high-powered speedboats far out to sea.

Faced with patrols by EU, Nato and US warships, the pirates moved into new waters. The attack on the Sirius Star occurred far beyond the 250 nautical miles from shore recommended by the International Maritime Bureau.

Residents of Harardheere reported seeing two dhows sail out towards the captured tanker yesterday. About 18 men boarded, carrying food and qat, the narcotic leaf chewed by many Somali men.